(520) 
point is the difference in parallax (dp). This distance should be measured and 
averaged to the closest 0.1 millimeter. 
|l. Measure on the transparency the distance from the photo center of the 
right-hand photograph to the second marked photo center of the left-hand 
photograph to the closest 0.1 millimeter. This is the adjusted photo base (Du) 
based on the elevation of the upper point and is equal to the factor (bi + dp). 
m. Draw on the transparency a pencil line from the photo center marked 
from the right-hand photograph to the upper point marked from that photo- 
graph. (Actually this line need extend but several centimeters from the upper 
point towards the photo center). Also draw the line from the second point 
marked for the photo center of the left-hand photograph to the other upper 
point marked on the transparency. The intersection of these two lines 
represents the corrected position of the upper point. 
n. Measure the distance on the transparency from the lower point 
(marked from the right-hand photograph) and the corrected position of the 
upper point to the closest 0.1 millimeter. This distance is the corrected hori- 
zontal distance. 
o. The tangent of the angle of dip may now be calculated, as shown in 
equation III, using the four factors of difference in parallax, adjusted photo 
base, corrected horizontal distance, and focal length. Actual calculations may 
be avoided if a nomogram is used. 
3. Measurement of Dip of Strata Exposed as Bedding Trace. 
a. Same as steps a and b as described above. 
b. Using the stereoscope, locate and mark with a needle point, welldefined 
upper and lower points on a bedding trace on the right-hand photograph. 
These points should be as close to each other in the direction of strike as 
possible, and at the same time, as far from each other in direction of dip as 
possible. 
c. Same as steps d through m, as described above. 
d. Under the stereoscope, on either photograph. place a small strip of 
clear acetate (about 1.5 centimeters by 7 centimeters on which a fine ink line 
has been ruled lengthwise), oriented so that the line passes through the lower 
point, and parallels the strike. Fix in place on the photograph by scotch 
drafting tape. 
e. Lay the transparency on the same photograph, oriented so that the 
ruled line passes through the photo center and transferred center; if the right- 
hand photograph is used, so that the photo center points coincide; if the left- 
hand photograph, so that the photo center points are oriented as in step 1, as 
described above. 
f. Measure on the transparency the distance at right angles from the line 
passing through the lower point to the corrected position of the upper point. 
This is the corrected horizontal distance. 
g. Calculate the angle of dip as described in step o above. 
C. Alternate Procedures to Measure the Difference in Height (dh). 
1. General. 
Alternate procedures for determining the difference in height (dh) between 
the upper and lower points depend upon a different method of measuring the 
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40